Anxiety disorders are extremely prevalent among children and adolescents, affecting 10-20% of school-aged youth. Cognitive behavioral therapy (CBT) is an efficacious treatment for child anxiety disorders, but approximately 40-50% of youth do not respond fully to treatment. It is not clear why these children and adolescents do not achieve full benefits from CBT. One possibility is that they don't apply the skills learned in therapy during their daily lives, either because they don't understand the concepts and skills well enough, or they don't remember to use the skills in real-world anxiety provoking situations. Recent advances in interactive mobile health (mHealth) technologies offer tremendous potential to enhance mental health treatments by improving skill acquisition and utilization in real-world contexts. Technology-enhanced therapeutics may be particularly fruitful in working with youth, given children and teens' comfort with technology and the large amount of time they spend engaged with technology. mHealth technologies may also help to improve youths' engagement in treatment by incorporating motivational messages from therapists, providing fun and interactive ways to learn new skills (i.e. games), and incorporating digital rewards to reinforce skill use and practice. We will refine and test an Ecological Momentary Intervention called SmartCAT (Smartphone-Enhanced Child Anxiety Treatment) that will provide daily opportunities for skill use between CBT sessions during real-world emotional experiences. SmartCAT consists of an app and an integrated clinician portal connected to the app for secure 2-way communication. The app and portal both contain a secure messaging interface and a digital reward bank to track points that the patient can earn by using the app. The app also contains a multi-media library for storing and accessing treatment relevant materials. In the present study, we will develop new interactive skillbuilder modules that can be activated by patients and/or triggered via GPS in pre-specified locations or at prespecified times, including an in vivo skill coach module to reinforce skill utilization, a series of interactive games and activities to reinfoce skill acquisition, and a customizable home challenge module to facilitate home-based exposures. In the development phase of the study (Phase 1), we will apply user- centered design principles to develop these new features based on feedback from 15 anxious youth ages 8-16. Feedback from youth, parents, and therapists will be incorporated and revisions will be made to the SmartCAT program for beta testing in Phase 2. Phase 2 will test the feasibility and acceptability of delivering CBT + SmartCAT in 20 anxious youth ages 8-16 with diagnoses of generalized anxiety disorder, separation anxiety disorder, and/or social phobia. Phase 2 will also provide initial data on whether SmartCAT effectively engages its target of improved CBT skill use, and whether better skill use is associated with better treatment response. This approach is consistent with treatment development pathways recommended in the R34 mechanism and also responsive to strategies outlined in the NIMH Strategic Plan for improving personalization of treatments.